1. Field of the Invention
This invention relates to a method for imaging nuclear magnetic resonance (to be referred to as "NMR", hereinafter) signals by using a non-linear magnetic field gradient which signals carry information on distribution of nuclear magnetic substance, such as hydrogen (H), fluorine (F), sodium (Na), carbon (C), phosphorus (P), and the like elements, in a body being measured (to be referred to as a "target", hereinafter). More particularly, the invention relates to a method of imaging NMR signals, in which the phase of nuclear magnetization at each local spot in the target is encoded by using a specifying magnetic field with a non-linear gradient, such as a gradient along a quadratic curve, of the magnetic field intensity, and NMR signals with different degrees of contribution from nuclear magnetizations at different localities are obtained by scanning the specifying magnetic field, so as to facilitate computer processing of the NMR signals for imaging, whereby an NMR image of an entire target can be formed efficiently in a short period of time without necessitating generation of any strong non-linear magnetic field gradient.
2. Description of the Prior Art
As a conventional method for imaging NMR signals carrying information on the inside of a target, an NMR computed tomography (to be referred to as the "NMR-CT", hereinafter), using a linear magnetic field gradient is known. More specifically, a conventional method for NMR imaging is based on the fact that when the NMR signals are detected under the presence of a linear magnetic field gradient, the spectrum of the NMR signals becomes proportionate to a projected image of the spin distribution in the target, the projected image being taken in a direction perpendicular to the direction of the magnetic field gradient. The conventional method forms an image of NMR-CT by applying the same algorithm as that of the x-ray computed tomography (to be referred to as the "x-ray CT", hereinafter) to a plurality of such projected images detected by varying the direction of the linear magnetic field gradient to a number of orientations.
Another conventional method for NMR imaging is based on the fact that when free precession of nuclear magnetization is effected under the presence of a linear magnetic field gradient, the phase of the nuclear magnetization varies in proportion to the spatial position in the target, and that quantities proportional to one-dimensional Fourier transformation (FT) of the spin density distribution can be obtained by systematically varying the timing and the value of the magnetic field gradient for effecting the free precession of the nuclear magnetization. Thus, in the so-called FT zeugmatography or spin warp method of the prior art, which has been clinically tested, data proportional to two-dimensional or three-dimensional Fourier transformation of the spin density distribution are produced by switching the direction of the magnetic field gradient in orthogonal two directions or orthogonal three directions, and imaging is effected by taking the inverse Fourier transform of the data.
Various NMR-CT approaches of the prior art, which use the linear magnetic field gradient, have a shortcoming in that a strong magnetic field gradient is necessary to achieve a high spatial resolution in the obtained image, and the strong magnetic field gradient tends to cause expansion of the NMR spectrum and reduction in the height of the NMR spectrum, so that the S/N ratio of the obtained image is reduced with the increase in the strength of the linear magnetic field gradient for a given intensity of the static magnetic field. Accordingly, in order to improve the resolution of the obtained image without deteriorating the S/N ratio thereof, it becomes necessary to increase the static magnetic field intensity. Then, the conventional NMR-CT using the linear magnetic field gradient has a major technical problem or shortcoming in that a strong static magnetic field with a high homogeneity over a wide range and a high stability must be provided.
Recently, it has been proposed to specify a small restricted region of the target for measurement by using a non-linear magnetic field gradient or the so-called specifying magnetic field. For instance, the inventors proposed methods for collection of information only from the proximity of the center of the specifying magnetic field with non-linear gradient based on the difference in the NMR frequency, as disclosed in the inventors' Japanese Patent Laid-open Publications No. 103,693/1974, No. 127,785/1976, and No. 133,192/1979 relating to methods for imaging NMR signals.
The conventional method for imaging NMR signals using a non-linear magnetic field gradient has a shortcoming in that a specifying magnetic field to be used therein is required to have a strong non-linear gradient to specify a small region for measurement and to clearly define the boundary of such small region, and coils with comparatively large ampere-turns are necessary for the generation and scanning manipulation of the specifying magnetic field with such strong non-linear gradient. The requirement for coils with large ampere-turns is a kind of limitation to the formation of a large imaging device, especially a large magnetic field generator.
Besides, the above-mentioned conventional method for imaging NMR signals by using the non-linear magnetic field gradient has another shortcoming in its slowness in producing two- or three-dimensional images. More particularly, the specifying magnetic field with the non-linear gradient specifies a small region for each direct measurement, so that each application of the high-frequency (to be referred to as "HF", hereinafter) pulse magnetic field for detection of nuclear magnetization yields measured data from only a small region, and it takes a long time to collect sufficient data for imaging the entire target on a two- or three-dimensional basis. As compared with the NMR-CT methods using the linear magnetic field, the methods using the non-linear magnetic field may be left behind as far as the speed of collecting the measured data is concerned.